Proper construction of the vertebrate central nervous system (CNS), culminating in the formation of functional circuits, depends on the appropriate number of neurons reaching a specific location. The migration of neurons from their birthplace to their final destination is a key process in neuronal development. While radial migration of neurons along glial fibers has been known, it has recently become apparent that migration of neurons orthogonal to glial fibers, a process termed tangential migration, plays an important role in the formation of the CNS. Interneurons use first tangential then radial migration to arrive at their final position in the 01 factory bulb. In the first stage, neurons move along a tangential migratory route from the anterior region of the subventricular zone (SVZa) to the entrance of the bulb. Subsequently, the neurons follow a radial migratory path to their laminar position within the olfactory bulb. Unlike other brain regions that contain small dispersed groups of tangentially migrating neurons, the olfactory bulb interneurons represent a large population of postnatally derived neurons that follow a precise tangential migratory route, making this an invaluable system for the identification and characterization of the molecules that guide cells during the tangential migration process. Since proteoglycans have been shown to play an important role in the migration of neural crest and primordial germ cells, immunohistochemistry or in situ hybridization using probes specific for individual cell surface or extracellular matrix molecules will be used to identify molecules involved in the tangential migration process. Similarly, the Ncaml8O mutant mouse, which specifically lacks the highly sialylated 180 kDA isoform of the neural cell adhesion molecule (N-CAM) and in which migration of interneuron precursors to the bulb is greatly reduced will be examined for expression of either a cell adhesion molecule or substrate adhesion molecule that could block cell migration.